A room at a guest house on Angles Road in Streatham was destroyed by a fire as a man allegedly tried to set his testicles alight.
The man, believed to be suffering from mental ill health, was taken to hospital in the incident which saw his room at the bed and breakfast destroyed by the fire.
It is understood he sustained injuries to one of his arms.
The fire brigade were called at 11pm.
A spokeswoman from the Blue Watch said: "The room was completely burnt out but luckily the door was closed so there was no damage to the rest of the building.
"The police are investigating the cause of the fire."
The bed and breakfast is still open and no other occupants were harmed by the fire.
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